1. Field of the Invention
This invention relates to the sensing of surface contaminates and trace gases in the atmosphere and more particularly to the sensing of such materials at remote locations. In one embodiment, the detection results from fluorescence initiated by illumination with laser light, and in another arrangement, differential absorption of the substance serves as the basis for detection. Local or remote sensing may be accomplished by examination for fluorescence produced by a reaction product of a chemical reaction between the substance to be detected and a selected reagent.
2. Description of the Prior Art
Laser systems have been explored for the remote detection of chemical substances since early in the 1960's. One method has made use of fluoresence by the substance of interest initiated by laser light. In this method, the laser wavelength is selected to be near or at the peak of an absorption resonance in the trace gas or chemical element to be detected. This absorption resonance is selected to be one that causes strong fluorescence. In one such arrangement, a pulse of laser light is transmitted in the atmosphere in the area where the presence or absence of the particular substance is to be determined. A receiver, directed toward the radiant laser light, is arrranged to respond to the fluorescent radiation. For discriminating against spurious background fluorescence, a differential method is used. In this case, two successive pulses of different wavelengths are transmitted: one lying at a wavelength near or at the peak of the absorption line of the substance to be detected and the other at a wavelength appreciably removed from the absorption line. The fluorescent signals from the two pulses would appear predictably at different intensities making it possible to discriminate against spurious background fluorescence. In general, however, because the fluoresence from the substance at a remote location occurs over a solid angle of 4 pi radians, only a relatively weak signal is available to the receiver. This is equally true whether the fluorescence is from a reaction product or when the presence of biological agents is to be detected by the natural UV fluorescence. Such arrangements require a high energy laser beam and a sensitive receiver: the greater the distance between the substance to be detected and the location of the sensor unit, the greater must be the intensity of the laser light and the sensitivity of the receiver. The receiver usually requires narrow-band filters and a narrow field of view to discriminate against incident daylight. With the best of equipment, such arrangements are effective only over limited distances.
Contaminates in the air or on a surface have been detected by spraying a reactant chemical, selected to produce a reaction product having strong fluorescence, into the air or onto the surface and illuminating the area with laser light to cause fluorescence of the reaction product. Such arrangements have been limited to laboratory use and generally have been unsatisfactory for field use.
The detection of nerve gas and other chemical and biological agents is of great importance in military and other situations, but remote sensing of such agents has not been particularly sucessful. One problem, in addition to the problems of long-distance laser illumination and fluorescence detection, is caused by the fact that some substances do not have a strong fluorescence intensity that is readily detected. Another factor is that objectionable chemical agents may be camouflaged by releasing a chemical with similar fluorescence properties. In some instances, it is possible to provide a chemical reactant capable of reacting with the suspected contaminate to produce a reaction product selected to be one having a strong fluorescence.
A specific example is a remote sensor for agent specific detection of nerve gas soman (GB) with indole and sodium perborate in a mixture of acetone and water. From an early investigation in 1957 [See B. Gehauf and J. Goldenson, Anal. Chem., 29, 276 (1957)], it is known that in a two step process, the reaction produces indoxbyle, which fluoresces strongly when subjected to laser radiation in the 350 nm wavelength region. A XeF laser at 350 nm, or a nitrogen laser at 337 nm, can be used. The fluorescence spectrum peaks at 450 nm and has a width of about 50 nm. See Alan Hartford, Quarterly Progress Report AP-4-82:109, July 18, 1982, Page 1, Los Alamos National Laboratory, University of California, Los Alamos, N.M.